The Heliotrygon stingray - with a smaller relative in an aquarium - can measure up to a metre in diameter, but has a minimal sting. David Webber/www.belowwater.com

Crocodile hunter Steve Irwin's well-publicised death in 2006 from a stingray attack only heightened public fascination with the animals. Their stings, located on the tail, are sharp, with backward-pointing serrations and venom from mucous secretions at their bases. Most human stings result from bathers stepping on an unsuspecting stingray, innocently resting on the sea bottom, which produces a defensive reflex of the tail forward. But stingrays are not limited to saltwater.

Although the Potamotrygonidae family of freshwater stingrays has been known since 1843, two remarkable new species from South America were recently described. Both are nearly circular in shape, the most round disc of any of the current tally of approximately 200 known species of stingrays. The genus was named Heliotrygon, a combination of helio – sun – and trygon, which means ray. Although the two species are new to science, specimens of the genus have shown up in the aquarium fish trade under common names such as "round" or "pancake" rays.

These species have tiny eyes and live in murky waters of the central channel areas of Amazonian rivers, coming closer to shore mostly at night to feed. Even though they have reduced mouths that are located away from their snouts, they eat fish almost exclusively. You may be asking how a flat ray, with a tiny mouth located on the underside of its body, could catch fish. Using their "lateral line system", sense organs that can detect movements and pressure changes in surrounding water, they spot their prey, thrust upward, lift the snouts and quickly open their small but very strong mouths to swallow a fish almost whole. One specimen examined by x-ray had a large catfish in its gut, spines and all.

Although these rays are the last thing seen by many fish, they present no danger to us. The stings of Heliotrygon gomesi are among the smallest of all stingrays. A specimen 70cm across has a sting only about as long as the fingernail of your pinky and only one-tenth as wide, incapable of inflicting much damage.

Dr Marcelo de Carvalho of the Universidade de São Paulo, one of the scientists who discovered the new species, says that the discovery highlights two important aspects of modern biodiversity science: "That we still have much to discover and that there are too few researchers devoting themselves to this type of research." De Carvalho says that fieldwork over the past decade has led to the discovery of 15 new species in this specific stingray family. He stresses: "We're talking here of large stingrays, up to one metre wide. There are a mere 10 research groups working on the taxonomy and diversity of sharks and rays worldwide, even though much collecting and fieldwork is needed to understand the diversity of the group. Part of the challenge is remarkable genetic diversity: no two specimens, even those collected side by side and of the same gender, size, and species, are entirely alike. This great variation has to be properly understood before we can know the true diversity of the group."

Ecologically, because stingrays have internal fertilisation and reproduce slowly compared to many freshwater fishes, they are more easily threatened by environmental degradation or illegal harvesting by the aquarium industry. "They may be good indicators of general environmental health, as top predators usually are," says de Carvalho. "And all that rests of the shoulders of good descriptive taxonomy and biodiversity science."

Quentin Wheeler is director of the International Institute for Species Exploration, Arizona State University